Two techniques widely used in the fabrication of printed circuit boards are the additive process and the subtractive process.
In the manufacture of printed circuit boards by the subtractive process, there is adhered to the surface of a copper-clad laminate one surface of a dry, photoresist film; the other surface of the film is adhered to a film support. The photoresist layer is thereafter exposed to light according to a predetermined pattern. The support film is thereafter removed and the copper layer is exposed by washing away the unexposed areas of the photoresist layer. The exposed copper layer is then etched away by chemical milling.
In the additive process, there is adhered to the surface of a substrate (which may or may not be copper-clad) one surface of a dry photoresist film; the other surface is adhered to a film support. The dry photoresist layer is thereafter exposed to a predetermined light pattern and the support film is thereafter removed. The substrate is exposed by washing away the unexposed areas of the photoresist layer. At this point, the substrate is treated with appropriate sensitizers and metalizing solutions, whereby a layer of metal, usually copper, is applied to the substrate.
The dry photoresist films are typically formulated to contrast in color with the substrates with which they are used. Since they generally do not change color, it is extremely difficult to inspect the product after exposure to light but prior to removal of the unexposed areas of the photoresist film to insure that the light mask has been properly placed and to be sure that there are no imperfections or defects in the exposed photopolymerizable layer. It is extremely useful to have a dry photoresist film which visually indicates where the film has been exposed to light so that detection of defects can readily be made prior to developing the film and etching the copper-clad panels or building up a layer of metal on the exposed substrate.
It would also be useful to have a liquid photopolymerizable system that is colorless or lightly colored in the unexposed state and which both crosslinks and becomes intensely colored upon exposure to actinic light.